This invention relates to a system for exciting a step motor which is rotated in response to input signals.
Step motors are high in rotational angle control accuracy, and can be operated in an open loop control which requires no feedback. Therefore, they are extensively employed as driving sources for various mechanisms. In driving the step motor, for every stepping operation thereof it is necessary to obtain a torque which is sufficient for the given inertial moment. Especially when a high speed drive is required, the torque must be considerably high.
FIG. 1 shows a drive circuit according to a conventional chopper excitation system for driving a step motor at high speed. The circuit is connected to the four-phase exciting coils 1A, 2A, 1B and 2B of the step motor. First ends of the exciting coils 1A and 2A are connected, through the emitter and collector of a chopper transistor 3A adapted to control the supply of exciting current, to a power source line 4. The other ends of the exciting coils 1A and 2A are grounded through the collectors and emitters of transistors 5A and 6A, adapted to excite these coils, and through a current detecting resistor 7A for detecting excitation currents flowing in the exciting coils 1A and 2A. One terminal of the current detecting resistor 7A and an input terminal 8A, for inputting a reference voltage V.sub.R, are connected to the input terminals of a comparator 9A, respectively. The output terminal of the comparator 9A, which is adapted to output a comparison result, is connected to the base of the chopper transistor 3A.
The arrangement of circuit elements for exciting the remaining exciting coils 1B and 2B is similar to that for exciting the coils 1A and 2A. Therefore, the circuit elements for the exciting coils 1B and 2B are designated by corresponding reference numerals with the suffix character "B", with the preceding description being applicable.
In the drive circuit thus organized, control signals are applied to the bases of the transistors 5A, 6A, 5B and 6B, to render these transistors conductive with predetermined timing. When the transistors 5A, 6A, 5B and 6B are selectively rendered conductive by the control signals, the coils 1A, 2A, 1B and 2B are excited, respectively. In the initial exciting operation of each of the coils 1A, 2A, 1B and 2B, the voltage drop across the current detecting resistor 7A or 7B due to the exciting current is small. Accordingly, in this case, the voltage V.sub.O supplied to the comparator 9A, or 9B is low, and the chopper transistor 3A or 3B is maintained conductive. In the drive circuit, a relatively high voltage is applied to the chopper transistors 3A and 3B through the power source line. Accordingly, in the initial state as described above, the exciting current is increased quickly. As the exciting current is increased, the voltage drop across the current detecting resistor 7A or 7B is increased, and accordingly the voltage V.sub.O is increased. When the voltage V.sub.O reaches the reference voltage V.sub.R, the corresponding chopper transistor 3A or 3B is rendered non-conductive. Thereafter, on-off operation of the chopper transistor 3A or 3B is repeatedly carried out according to the relationship between the voltage V.sub.O and the reference voltage V.sub.R, so that the operations of the exciting coils are controlled in a constant current mode.
Thus, according to the conventional step motor exciting system, the currents of the exciting coils are allowed to rise quickly. Therefore, a torque high enough to drive the step motor at high speed can be provided.
In the case where the step motor is driven by one-two phase excitation according to the above-described system, when two phases are excited simultaneously, the exciting current flowing in each exciting coil is two times that flowing in each exciting coil when one phase is solely excited. Accordingly, the magnetic energy which is produced in the step motor with one-phase excitation is much different from that which is produced in the step motor with two-phase excitation. Thus, in this case, the torque is nonuniform. Especially, where a load is applied to the step motor, the characteristic of the rotation angle with time is not linear as shown in FIG. 2, because the rotor rotation angle at every pulse signal supply period T.sub.1 become .theta..sub.2 in two-phase excitation, which is larger than .theta..sub.1, which is the rotor rotation angle with one-phase excitation. Accordingly, if the above-described system is employed with the step motor of an original reading mechanism as used, for instance, in a facsimile device, the scanning lines are irregularly spaced, with the result that the resultant image data are not correct.